1. Field of the Invention
The present invention relates to a printing head for ejecting ink, and an ink jet printing apparatus which performs printing by means of the printing head. More particularly, the present invention relates to electrical connections in the circuit boards, etc. of the printing head.
2. Description of the Related Art
Ink jet printers or the like, as apparatuses in which printing is performed by ejecting ink onto a printing medium, are widely used. Such an ink jet printing apparatus includes a printing head for ejecting the ink Besides, signals such as a head drive signal are exchanged between the printing head and an apparatus main body, and the electric power of a power source is supplied from the apparatus main body to the printing head.
FIG. 5 is an exploded perspective view showing a printing head in a prior-art example. The printing head shown in the figure is of the so-called “side shooter type” wherein ink is ejected in a direction perpendicular to a substrate of the printing head on which heaters or the like are disposed.
As shown in FIG. 5, a printing head 100 is generally made of a body 18, a printing element board 10, a supporting board 20 and a printed wiring board 30. The printing element board 10 has ink ejection orifices, a substrate which is formed with heaters for generating energy for ejecting ink from the orifice, and the like. The supporting board 20 is attached to the body 18 in a state where the supporting board supports the board 10. The printed wiring board 30 is disposed so as to surround the printing element board 10, and drive signals for ejecting ink, etc. and supply power are fed to the board 10 through wiring lines laid in the board 30. The body 18 is provided with an ink supply portion 180, an electrical connection portion 181, etc. The ink supply portion 180 has an ink tank IT detachably attached thereto, and the portion forms an ink supply passage from the attached ink tank to the printing element board 10. The electrical connection portion 181 comes into contact with an electrical connection portion of a carriage (not shown) when the printing head 100 is mounted on the carriage, whereby the electrical connections of the printing head 100 with the apparatus main body can be made.
The body 18 is so configured that the above portions such as the ink supply portion 180 are unitarily molded by, for example, a resin. The ink supply portion 180 has a recess portion 182G for receiving the supporting board 20 therein. The bottom of the recess portion 182G is a surface used as a joint surface 183 on which the supporting board 20 is bonded. As shown in FIGS. 6A and 6B, a part of the joint surface 183 is formed of the surfaces of block pieces 26 which are formed of, for example, an aluminum alloy. The block pieces 26 are disposed in a metal mold when the body 18 is molded with the metal mold, whereby they can be covered with the resin forming the body 18. The end of the ink supply passage 184 for introducing the ink from the ink tank IT is an opening at substantially the central part of the joint surface 183.
The substrate forming the printing element board 10 is a thin film of silicon material having a thickness of 0.5 mm-1.0 mm. Besides, the substrate is formed with an opening which communicates with the above-mentioned ink supply port 184 through the opening 20a of the supporting member 20, as shown in FIG. 5. In addition, heaters and partition walls dividing the heaters are respectively disposed in correspondence with a plurality of ink ejection orifices on both the sides of the opening 10c on the substrate. Thus, the ink supplied from the ink tank IT is fed into respective ink paths provided with the corresponding heaters.
As shown in FIG. 5 and FIGS. 6A, 6B, the printed wiring board 30 is electrically connected to the printing element board 10. The printed wiring board 30 has a printing-element accommodation portion 10B in which the printing element board 10 is accommodated for the electrical connections, and an input terminal portion 10A which is disposed in the electrical connection portion 181 of the body 18. The electrical connections between the printed wiring board 30 and the printing element board 10 are done using a TAB (tape automated bonding) method.
The supporting board 20, which is arranged between the printing element board 10 and the joint surface 183 of the recess portion 182G in the ink supply portion 180, is formed in the shape of a rectangular plate, as shown in FIG. 5 and FIGS. 6A, 6B. The supporting board 20 is formed of silicon which is the same material as that of the substrate forming the printing element board 10. Incidentally, this material is not restrictive, but the supporting board 20 can also be formed of a material which exhibits a linear expansion coefficient equivalent to that of the material of the printing element board 10, and a thermal conductivity equivalent to or higher than that of the material of the printing element board 10. By way of example, the material of the supporting board 20 may be any of alumina (Al2O3), aluminum nitride (AlN), silicon carbide (SiC), trisilicon tetranitride (Si3N4), molybdenum (Mo) and tungsten (W).
As shown in FIG. 6A, the supporting board 20 has a second joint surface 20Sa which is bonded onto the surface provided with the ink supply opening 10c in the printing element board 10, and a first joint surface 20Sb which is bonded onto the joint surface 183 of the recess portion 182G in the ink support portion 180. Besides, the supporting board 20 has a communicating passage 20a formed to extend in the lengthwise direction of this board 20, at a position which corresponds to both the ink supply opening 10c and the ink supply passage 184 provided in the joint surface 183. Further, the lengths of the shorter side and the longer side of the supporting board 20 are respectively the same as those of the shorter side and longer side of the printing element board 10, and the thicknesses of both the boards 20 and 10 are substantially the same.
In disposing in the body 18 the printing element board 10 to which the printed wiring board 30 is connected, the first joint surface 20Sb of the support board 20 is initially bonded to the predetermined position of the joint surface 183 by an adhesive. Subsequently, as shown in FIG. 6B, the second joint surface 20Sa of the support board 20 is bonded by the adhesive onto the surface provided with the ink supply opening 10c in the printing element board 10. Usable as the adhesive is, for example, one which has a low viscosity, which can form a thin bonding layer on the bonding surface, and which exhibits a comparatively high hardness after having been set.
In the printing head configured as described above, when heater drive signals corresponding to printing information are fed to the heaters of the printing element board 10 through the printed wiring board 30, the heater generates heat and thus an air bubble is generated in the ink, and thereby the ink is ejected by the pressure of the air bubble.
An example of the prior-art printing head as described above is shown in Japanese Patent Application Laid-open No. 07-144418(1995). However, the electrical connections of such a prior art between the printing element board and the printed wiring board has involved several technical problems as stated below.
In the first place, since the TAB method is employed as a method for connecting electrode wiring lines of both the boards, multi-layering of the electrode wiring lines in the printed wiring board is difficult, and the multi-layering is very difficult especially in case of arranging electrode wiring lines at a high density. Therefore, the electrode wiring lines of, for example, the above-mentioned flexible board (printed wiring board) are often laid as wiring lines of single layer. As a result, the number of the wiring lines in the flexible board has a certain limitation, and the sorts of signals which can be fed to the printing head through the flexible board are also limited.
In order to cope with the above drawback, it is considered to heighten a wiring density with the intention of increasing the number of wiring lines, but the intervals of the wiring lines need to be shorten accordingly (to, for example, 100 μm or less). Since, in this case, the wiring lines are formed at the very small intervals, a process for the formation of wiring lines sometimes becomes complicated.
Besides, since the electric power consumption of the printing head employing the above heaters is comparatively large (several tens W-about 100 W), the width of wiring line need to be maintained at a sufficient scale, in case of feeding the corresponding electric power without increasing the number of the wiring lines. This incurs the problem that the wiring board enlarges to increase the size of the printing head.
Secondly, in the TAB method, electrodes are connected to exposed electrode terminals at a connection part between the printed wiring board and the printing element board. Consequently comparatively rigid sealing needs to be performed by increasing an amount of a sealing material. As a result, the sealing material is deposited to form a convex portion on an ejection orifice surface of the printing head. In consequence, the spacing between the printing head and printing paper needs to be held at, at least, a certain magnitude, and the flight distances of ejected ink droplets enlarge to that extent and form a factor for hampering an improvement of a printing precision.
The above problems will be concretely explained with reference to FIGS. 7A and 7B below. These figures are views showing the layout of the electrical connections in the prior-art printing head unit shown in FIG. 5 and FIGS. 6A, 6B, and FIG. 7A is a sectional view taken along line VIIA—VIIA in FIG. 7B.
For the simplification of the description, FIGS. 7A and 7B show cases where three electrodes for feeding signals or electric power to the printing element board 10. According to this arrangement, three bump electrodes 40 are disposed on each of both the sides of the printing element board 10, and three electrode wiring lines 31 of a flexible board 30 are respectively connected to the three bump electrodes 40 by the TAB method. After the connections, respective connection parts are sealed with a sealant 70, as shown in the figures.
In the wiring layout, when the wiring lines 31 on the right side and the wiring lines 31 on the left side as are respectively symmetric with respect to the printing element board 10 feed the same signals or power levels, they are made common In prior-art cases, however, they have been made common on the side of the apparatus main body. More specifically, since the connections of the wiring lines 31 adopt the TAB method, these wiring lines 31 are difficult to be multilayered, and they cannot be made common in the vicinity of the connection parts with the printing element board 10. Therefore, the flexible board is mostly formed bearing the respective right and left wiring lines as the separate ones. As a result, the sorts of signals, etc. cannot be increased, or the width of the board itself needs to be enlarged for the purpose of ensuring predetermined sorts of signals, etc.
Further, as seen from FIG. 7A, comparatively rigid sealing is effected with the sealant 70, and the convex part of the sealant 70 are formed on the ejection orifice surface of the printing element board 10. For this reason, a sufficient distance needs to be set so that printing paper which is fed for printing may be prevented from touching the convex portion of the printing head. Herein, since the distance between the printing head and the printing paper is large in this manner, a landing precision of ejected ink droplets lowers, with the result that degradation in a printing quality is sometimes incurred.